[From the Philosophical Transactions for 1852, p. 463.]
ON THE CHANGE OF EEFRANGIBILITY OF LIGHT*.

[Read May 27, 1852.]
1. THE following researches originated in a consideration of the very remarkable phenomenon discovered by Sir John Herschel in a solution of sulphate of quinine, and described by him in two papers printed in the Philosophical Transactions for 1845, entitled " On a Case of Superficial Colour presented by a Homogeneous Liquid internally colourless," and " On the Epipolic Dispersion of Light." The solution of quinine, though it appears to be perfectly transparent and colourless, like water, when viewed by transmitted light, exhibits nevertheless in certain aspects, and under certain incidences of the light, a beautiful celestial blue colour. It appears from the experiments of Sir John Herschel that the blue colour comes only from a stratum of fluid of small but finite thickness adjacent to the surface by
* [In choosing the title of this paper, all that was intended was, to express the previously unsuspected origin of peculiar coloured lights which were exhibited by certain bodies, and which were in some cases matters of common observation, and had been subjected to examination by physicists. The capital fact now brought to light was that the colours in question were due to rays incident upon the body which were of a different refrangibility from those which constituted the colour observed. The title was meant to express a fact of observation, not any theory. It must have been from misapprehension on this point that a view as to the nature of the phenomenon has been attributed to me by more than one writer, a view which I have even been said to persist in, according to which the change of refrangibility takes place in the act of reflexion from the molecules of the body. Such a view is not mine, and never was, and is directly opposed to the views stated in the paper. The abstract, which was published some months before the paper, contains nothing about theoretical views, but for these refers to the paper. I can only conjecture that the view erroneously, though without hesitation, attributed to me was assumed to be mine in consequence of the title of the paper.]